This invention relates to methods for reducing shrinkage of a thermosettable polyester composition during cure, and to thermosettable and thermoset compositions employed therein.
Thermosettable polyester compositions possess many desirable properties such as a rapid cure rate, ease in handling, the facility to accept color and the ability to be easily modified for special characteristics. The resulting cured polyester compositions exhibit many useful properties such as dimensional stability, thermal stability and chemical properties such as resistance to attack by water and other solvents. When admixed with reinforcing materials such as glass fibers, these cured compositions have excellent mechanical properties such as high tensile and flexural strengths and moduli. Because of these properties, polyesters are used extensively in coatings, molding applications and reinforced plastic products.
Generally, thermosettable polyester compositions comprise an unsaturated polyester dissolved in a vinyl monomer such as styrene. Upon curing, the thermosettable polyester composition is cross-linked in the presence of a free radical initiator to form a thermoset composition by the reaction of the vinyl monomer with the unsaturated groups in the polyester. During this curing reaction the conventional polyester composition shows significant shrinkage. As a consequence, when the thermosettable polyester composition is subjected to compression or injection molding using a mold, there is a tendency for the shaped articles to become warped or cracked. Moreover, the shrinkage causes sink marks and irregularities on the surface of many shaped articles.
In view of these deficiencies, it has become common practice to incorporate in the unsaturated polyester and vinyl monomer mixture a small amount of a low profile additive such as a thermoplastic material, e.g., polystyrene and polyacrylate, to reduce shrinkage. See, for example, the paper given in 1970 at the 25th Annual Conference of the Society of the Plastics Industry (SPI) by Nussbaum et al., Section 6E, pages 1-5, "Smooth Surface Pre-mix and Sheet Molding Compound Technology". Upon curing of the thermosettable composition, the thermoplastic additive absorbs the volume lost by the resin. The exact mechanism by which the additive absorbs chemical shrinkage is not well understood. However, it is known that to be effective the additive must be insoluble in the thermosetting composition during cure such that two phases are present, one phase consisting primarily of the additive and styrene and the other phase consisting primarily of polyester and styrene.
In most cases, the use of a thermoplastic material as a low profile additive requires a two-package system. One of the packages generally contains the polyester-styrene resin solution and the other package generally contains a solution of the low profile additive; i.e., thermoplastic material in styrene. The two packages are insoluble and must be mixed prior to use. Cure must follow shortly thereafter, as the mixture upon standing will separate into two distinct macroscopic phases, one phase being additive rich and the other phase being rich in unsaturated polyester. When this separation occurs, remixing is required before further processing. For this reason, the two-package systems are often economically undesirable.
Several methods have been suggested for solving the problem of dispersion stability found in a two-package system. In one method, as discussed in U.S. Pat. No. 3,852,376, a polyethylene powder is employed as the low profile additive. This solves the dispersion stability problem. Unfortunately, due to the relatively poor compatibility between the unsaturated polyester resin and the polyethylene, the polyethylene exudes to the surface of the resin during cure. Consequently, the shaped articles lack in surface luster and uniformity. In a second method, as described in U.S. Pat. No. 3,852,376, a thermoplastic resin having an acid radical; e.g., a copolymer of methyl methacrylate and acrylic acid, is used as the low profile additive. The use of a thermoplastic resin of this type has been shown to improve dispersion stability. However, dispersion stability problems still exist after extended storage and the shaped articles prepared using the additive show pronounced nonuniformity of coloration. In yet another method, as presented in U.S. Pat. No. 3,836,600, adding a stabilizer, such as a poly(styrene-b-ethylene oxide) emulsifier to the thermosettable polyester composition containing a thermoplastic, low profile additive has been proposed. The resulting two-phase emulsion is stabilized by the emulsifier. Unfortunately, this stabilized emulsion requires an additional ingredient, i.e., the emulsifier, thereby increasing the costs of the molded article. Moreover, although stability of the emulsion is increased, stability problems still exist after extended storage.
In view of the stated deficiencies of the prior art methods, it remains highly desirable to provide an improved method of reducing the shrinkage of unsaturated polyester resins during cure.